I've found a link:
This issue was first mentioned to me back in the late '60's by a Nat'l Champ. So I took my .264 Win. Mag out to the range with extra firing pin springs; each about 1/4 inch shorter with compression weights in the 22, 19 and 16 pound range. Its factory 23-pound spring had been replaced by a 26-pound one. I was given these weakened ones by a 'smith (Elmer Shook, who sold me the 26-pound one) who built match rifles, then cut them off myself. At 1000 yards, as each weaker spring was used to shoot 5 rounds, elevation shot stringing increased. Went from about .6 MOA to 1.5 MOA.
Later, in a friend's underground 100-yard range, we did some muzzle velocity tests with weak springs in a .308 Win. chambered Win. 70. Weaker ones produced greater muzzle velocity spreads.
At the 1992 World Palma Matches, two guys on the US Team had elevation problems with the ammo issued. Both were given a new 26-pound spring from my tool box for the Win. 70's, then they shot better scores.
Some rifles have enough room on the firing pin to put a spacer between the cocking piece and the back end of the firing pin spring. This "renews" weakened ones up to about factory specs. I've done it many times with Win. 70's with a 3/8" long spacer.
At the Nationals in the early '90's, I talked with a rep from Federal about primers. He said they all need a small range of firing pin impact force as well as dimple depth to perform consistantly for best accuracy. And if not right, it often is the reason why a given load that wins matches and set records in some rifles will not do well in others; the start-up pressure curve's not right and the poor primer ignition/detonation is the cause. It's another reason why there's hundreds (thousands?) of favorite loads for a given bullet in a given cartridge.
In the link above, it also mentions what happens when the spring's too strong. I think 20% over factory specs is about the limit. For the 23 pound spec on Win. 70's, the limit would be about 28 pounds.